1. Field of the Invention
The present invention relates to an image recording apparatus, and more particularly to an electrophotographic printer.
2. Description of the Related Art
FIG. 2 illustrates the drive mechanism for a conventional art image recording apparatus.
The surface of a photoconductive drum 11 is uniformly negatively charged by a charging roller, not shown. An exposing unit such as an LED head array, not shown, illuminates the charged surface of the photoconductive drum 11 to form an electrostatic latent image thereon. Toner is supplied to a developing roller 12 from a toner cartridge, not shown, and is rubbed against the developing roller 12 by a developing blade, not shown, into a negatively charged toner layer. The toner applied on the developing roller 12 is then deposited to the electrostatic latent image to develop the electrostatic latent image into a toner image. The toner image is subsequently transferred by a transfer roller 13 to a print medium, not shown.
After the transfer operation, a small amount of toner is left on the surface of the photoconductive drum 11 and is removed by a cleaning roller, not shown, which is provided downstream of the transfer roller 13 with respect to the rotation of the photoconductive drum 11 and rotated in contact with the photoconductive drum 11.
A motor 14 in the form of, for example, a stepping motor or DC servo motor is provided. The rotation of the motor 14 is transmitted via a gear train to the photoconductive drum 11, charging roller, developing roller 12, transfer roller 13, and cleaning roller, so that the drum and rollers are rotated in directions shown by respective arrows.
A motor gear 21 mounted to a shaft, not shown, of the motor 14 is in mesh with a double gear 22 which in turn is in mesh with another double gear 23. The double gear 23 is in mesh with a gear 24 which is in mesh with a drum gear 15. Thus, the rotation of the motor 14 is reduced by a gear train constructed of the motor gear 21, double gears 22 and 23, and gear 24 before being transmitted to the drum gear 15.
The photoconductive drum 11 has the drum gear 15 at one longitudinal end thereof and a drum gear 16 at the other end. The developing roller 12 has a developing roller gear 17, and the transfer roller 13 has a transfer roller gear 18 at a location remote from the developing roller gear 17. The rotation of the photoconductive drum 11 is transmitted to the developing roller 12 via the drum gear 15, and to the transfer roller 13 via the drum gear 16.
The drum gear 16 and the transfer roller gear 18 are spur gears. The drum gear 15, developing roller gear 17, motor gear 21, double gears 22 and 23, and gear 24 are helical gears which transmit rotation smoothly.
With the aforementioned conventional art, the developing roller 12, transfer roller 13, and cleaning roller are in pressure contact with the photoconductive drum 11. Therefore, excess loads are exerted on the drum gears 15 and 16, developing roller gear 17, and transfer roller gear 18, so that the gears are deformed or twisted, resulting in variations in the pitches of the respective gears. Variations in pitches cause changes in rotation of the respective rollers, resulting in poor print quality.
The toner is charged by causing the developing roller 12 to rotate relative to the photoconductive drum 11 with friction therebetween. For this purpose, the photoconductive drum 11 and the developing roller 12 have different circumferential speeds, creating a frictional resistance between the photoconductive drum 11 and developing roller 12. The frictional resistance adds to the load on the drum gear 15 and developing roller gear 17, further causing the pitches of the drum gears 15 and developing roller gear 17 to change.
As a result, when a gray-scale image, not shown, is to be printed, variation in the rotation of developing roller 12 causes lateral stripes or lines in the print, resulting in poor print quality.
In order to deposit the toner on the developing roller 12 to the photoconductive drum 11, it is necessary to ensure that the photoconductive drum 11 has a substantially uniform area in contact with the developing roller 12 along the rotational axes of the photoconductive drum 11 and developing roller 12. However, when the rotation of the photoconductive drum 11 is transmitted to the developing roller 12, the drum gear 15 and the developing roller gear 17 tend to repel each other, causing a longer distance between the axes of the photoconductive drum 11 and developing roller 12.
FIG. 3 illustrates the relation between the drum gear 15 and the developing roller gear 17 of the conventional image recording apparatus. FIGS. 4A-4B illustrates a nip between the drum gear 15 and the developing roller gear 17.
Referring to FIG. 3, when the rotation of the photoconductive drum 11 is transmitted to the developing roller 12 via the drum gear 15 and developing roller gear 17, tooth surfaces S1 and S2 contact each other at an angle .alpha. equal to a pressure angle with respect to a line connecting center axes O1 and O2. The tooth surface S2 receives a drive force F in a direction at an angle of .alpha. with respect to a line between and tangent to pitch circles Ci1 and Ci2.
Thus, a component F.sub.Y of the drive force F, given by F.sub.Y =F.multidot.sin .alpha., acts in a direction parallel to the line connecting the center axes O1 and O2, so that the drum gear 15 and developing roller gear 17 repel each other. As a result, the distance between the center axes O1 and O2 becomes longer.
Due to the fact that the drum gear 15 and developing roller gear 17 are provided on longitudinal one ends of the photoconductive drum 11 and the developing roller 12, respectively, when the photoconductive drum 11 and developing roller 12 rotate, the photoconductive drum 11 and the developing roller 12 move away from each other at the right end so that the position of the longitudinal axis of the developing roller 12 center line is displaced from X to X' as shown in FIG. 4B. As a result, a nip between the photoconductive drum 11 and the developing roller 12 is not uniform along the lengths of the photoconductive drum 11 and developing roller 12. The nip is n at the left ends of the photoconductive drum 11 and the developing roller 12 but n-.delta.n at the right ends.
Less toner is charged with a decreasing size of the nip, so that the amount of toner deposited to the photoconductive drum 11 decreases along the length of the photoconductive drum 11. A decrease in the amount of toner causes lower density or an absence of toner in print. Insufficiently charged toner left on the photoconductive drum 11 after transferring is difficult to completely recover from the photoconductive drum 11. Such insufficiently charged residual toner builds up in the form of lines or stripes on the surface of the photoconductive drum 11 and may adhere to another print medium, thereby exposing the print medium to contamination.